One way to protect yourself from big losses is to buy 'put' options on the stock you hold. A put is basically a contract with someone, that says that they will purchase X amount of stock at Y price anytime between now and a set date in the future.
As example, lets say you own 100 shares of AAPL that you bought at $45 per share. Right now you're 'up' roughly $22 per share or $2200. But, you don't want to get caught in a freefall a la 2000-2001. Lets say you don't want to absorb anything below $60 for the next 6 months. You can purchase 1 contract (1 contract = 100 shares) of July AAPL puts at a strike of 60.00 for 4.40 per share ($440 for the contract).
What happens is this. If the price of aapl say, goes back to $45, then your 100 shares that you hold are now worth $4500 (back where you started). However, the contract you hold for 100 shares is also now worth the difference in market value vs the contracted sale price, which si 60-45=15 per share x 100 shares (the contract size). Basically, your put contract will be worth $1500.
The end result would be, you have $4500 in shares + contract worth $1500 = $6000. You also spend $440 on the contract originally so you're actually sitting at $5560.
The downside to this is that the contract is only good until the 3rd friday in July. It basically means you're not going to lose anything more if the stock goes below 60 between now and July, but you can still lose more after july.
There are lots of ways you can juggle this. For example, you can buy January 2008 options for 3.80 per share. 100 shares is $380. That would insure that there is no way you could lose money from your original $45 / share investment between now and January 2008 (other than the $380 you put in).